The present invention generally relates to the electronic stimulation of osteogenesis and in particular to implantable bone growth stimulators.
The electronic stimulation of bone growth (or osteogenesis) has been known for a period of time, as evidenced by U.S. Pat. No. 3,842,841, issued to Brighton, et al. on Oct. 22, 1974, entitled: "Constant Current Power Pack For Bone Healing and Method of Use". In the patent, it is disclosed that the provision of a cathode inserted at or near a bone fracture site and connected to a Direct Current source with an anode taped to the patient's skin will stimulate bone growth at the fracture site. A publication by Zimmer.USA, Inc., literature No. B-2360-1, revised in September, 1979, available from Zimmer.USA, 727 North Detroit Street, Post Office Box 708, Warsaw, Ind. 46580, provides a detailed description of Direct Current bone growth stimulation and one advantageous system to implement such stimulation. In such a system, the bone growth stimulating current and/or the cathode is inserted transcutaneously into the region of the fracture site. This insertion of a foreign object through the skin provides a possible point for infection of the skin and fracture site.
Thus, it is desirable to be able to totally implant a bone growth stimulation device. Many heart pacemakers are implanted and powered by an external current supply. Of course, the current level required to operate a heart pacemaker is an order of magnitude less than that current required for bone growth stimulation. Further, while the pacemaker can effectively utilize Alternating Current, the bone growth stimulation process, as embodied in the referenced patent, requires Direct Current.
These conditions have been met in the past through the utilization of a battery powered implantable bone growth stimulator, wherein the battery pack, power supply, and associated connections are totally implanted under the patient's skin, allowing the skin to heal thereover, preventing the possibility of infection. Unfortunately, a battery has a limited life and many patient's after the limb has healed to a certain extent, do not complete follow-up visits to their doctor to permit the doctor to remove the bone growth stimulator. Consequently, the battery pack may remain implanted for many years. Eventually, other problems can be created by the decomposition and/or deterioration of the battery pack, which may require extensive surgery at some point in the future. The elimination of the battery has been considered by researchers in the bone growth stimulation area as a positive improvement. However, it was conventionally thought that it would be necessary to have at least a buffer battery, or a large buffer capacitor of some sort, to provide a needed reference voltage for the constant current generator which powered the cathodes located in the fracture site.